PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 401 
Types—Holotype: 110341b; figured paratypes: 110341a,c,d. 
Horizon and locality—Dryden formation in Tennessee: I mile west of Lone 
Mountain, Tazewell (T.V.A. 154-NE) Quadrangle. 
Discussion—In ornamentation and outline this species is very much like 
D. transversa but it differs in having a much more convex brachial valve. Young 
specimens of D. transversa comparable to the adults of D. transversoides have 
a much less convex brachial valve than that of the latter species. 
DINORTHIS VENUSTA Cooper, new species 
Plate 58, A, figures 1-3; plate 71, H, figures 41-50 
Shell of about medium size for the genus, wider than long; subrectangular in 
outline ; cardinal extremities obtuse; sides broadly rounded ; anterior somewhat 
truncated; anterior commissure broadly uniplicate; surface strongly costate, 
about 34 on a large specimen ; costae narrowly rounded in the young stages but 
becoming broad and low anteriorly. 
Pedicle valve unevenly convex in lateral profile; the posterior half gently con- 
vex but the anterior half flattened; anterior profile flat; umbonal region gently 
swollen in the posterior half; anterior half gently depressed to form a faint but 
wide sulcus ; lateral areas flattened except for the posterolateral extremities which 
are deflected slightly toward the brachial valve; interarea short, strongly apsa- 
cline ; muscle field small but of the usual shape for the genus. 
Brachial valve strongly convex in lateral profile with the maximum convexity 
in the median region; anterior profile strongly convex with the median region 
swollen and the lateral slopes steep ; umbonal region well rounded ; median region 
inflated ; sulcus originating just anterior to the umbo, shallow and poorly defined 
to a point just anterior to the middle where it is elevated to form an ill-defined, 
broad fold ; lateral slopes steep. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
EVOLGEY Pe hres e s's os.c 18.6 19.1 24.3 18.6 9.0 
Paratype (110286b).. 16.7 16.4 20.9 16.0 7.5 
Type.—Holotype: 110286a ; figured paratypes: 110286b, 116978a-c. 
Horizon and locality—Shaly bed in upper Red Knobs formation in Tennessee: 
2 miles southeast of Kiser, Meadow (T.V.A. 139-NW) Quadrangle; 3 miles 
northwest of Madisonville, near fault, Sweetwater (T.V.A. 131-SW) Quad- 
rangle. 
Discussion.—This species is characterized by its compact, subrectangular out- 
line and the strong costae. It is very similar to D. atavoides in its exterior de- 
tails, but there are nevertheless many differences. D. venustus has a flatter 
pedicle valve which is not medianly inflated as in D. atavoides. Furthermore, the 
sides of the Red Knobs species are flatter and the sulcus is somewhat broader 
and more poorly defined. Dinorthis venustus is a less transverse shell than 
D. atavoides. The profiles of the brachial valves are similar, but that of D. venus- 
